"Ok, how about using the Noesis design, but like you said substituting with dual 18s, and a larger horn? (same compression driver)."
the coaxial driver in the noesis can be crossed at 450hz and that is low enough for good 18's to integrate with.
it is actually one concept that had been mentioned in chop's big seosr thread. if you get a chance, that whole thread is worth reading. there are many design tips and nuggets that will benefit anybody looking to create absolute monster mains.
another option would be to do something like scott is shooting for and put a quad set of 2226 drivers on the bottom. destroyers his will be...